The present application relates to the field of imaging modalities. It finds particular application to imaging modalities that can employ photon counting techniques (e.g., such as image modalities that employ x-ray and/or gamma radiation). For example, medical, security, and/or industrial applications may utilize a computed tomography (CT) scanner comprising photon counting channels to count the number of photons that are detected by respective channels. Based upon the number of photons detected, one or more images providing a two-dimensional and/or three-dimensional representation of an object under examination may be generated therefrom.
Today, CT and other imaging modalities (e.g., single-photon emission computed tomography (SPECT), mammography, digital radiography, etc.) are useful to provide information, or images, of interior aspects of an object under examination. Generally, the object is exposed to radiation comprising photons (e.g., such as x-rays, gamma rays, etc.), and an image(s) is formed based upon the radiation absorbed and/or attenuated by the interior aspects of the object, or rather an amount of photons that is able to pass through the object. Generally, highly dense aspects of the object absorb and/or attenuate more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or metal, for example, will be apparent when surrounded by less dense aspects, such as muscle or clothing.
Radiographic imaging modalities generally comprise, among other things, a detector array comprised of a plurality of channels that are respectively configured to convert radiation that has traversed the object into signals that may be processed to produce the image(s). The channels are typically “charge integrating” and/or “photon counting” type channels (e.g., the imaging modality operates in charge integration mode, photon counting mode, or both).
Charge integrating channels are configured to convert energy into signals (e.g., current or voltage signals) that are proportional to an incoming photon flux rate. Respective signals may then be integrated over a time period (e.g., at times referred to as a measurement interval), sampled, and digitized. While this type of channel is widely used, there are several drawbacks to such channels. For example, charge integrating channels are generally not able to provide feedback as to the number and/or energy of photons detected. As another drawback, there is a lower limit of detection defined by noise in the channel such that a channel with little to no incident radiation may produce some signal due to thermal and/or analog read noise (e.g., produced by the detector array and/or readout components). It will be appreciated that as a result of this lower limit, the dose of radiation that is applied to an object under examination is generally greater than the dose of radiation that may be applied to the object if the channels are of a photon counting type.
Photon counting channels are configured to convert energy into signals that are proportional to the energy of a detected photon (e.g., at times referred to as a detection event). Thus, ideally, signals produced by respective channels generally comprise one or more current and/or voltage pulses, for example, respectively associated with a single detection event. A controller may then be used to determine the location and energy of respective detection events, accumulate the detection events occurring during a measurement interval (e.g., an “acquisition view”), digitize the information, and/or process the digital information to form an image, for example. It will be appreciated that there are numerous advantages to photon counting channels over charge integrating channels. For example, the counting of photons is essentially noise free (e.g., apart from inherent photon shot noise). Therefore, a lower dose of radiation may be applied to the object under examination. Moreover, photon counting channels generally allow for energy or wavelength discrimination. Therefore, images resulting from radiation emitted at different energy levels may be obtained at the same or substantially the same time, for example.
While photon counting detector arrays (e.g., detector arrays comprising photon counting channels) have numerous advantages over charge integrating detector arrays, photon counting detector arrays have not been widely applied in some imaging modalities (e.g., such as CT) that have a high photon emission rate. One reason photon counting detector arrays have not been widely adopted is due to saturation issues (e.g., pulse pileup). Saturation occurs when photons are detected at a rate that causes one or more channels to be unable to return to a normal state after the detection of a photon before another photon is detected. That is, stated differently, respective channels of the detector array are configured to emit an electric pulse when a photon is detected. The electric pulse is intended to be merely indicative of a single detection event and is typically counted as a single detection event. However, when two or more photons are detected by a channel in close temporal proximity, the emitted electric pulse may be indicative of two or more detection events (e.g., because the channel was not able to return to an electrically normal state prior to the detection of a second, third, etc. photon, causing the pulses to pile-up). Because the counters are typically configured to count a single detection event per pulse, the counters may mistakenly count merely a single detection event when the pulse is, in fact, indicative of two or more detection events. Thus, due to this phenomenon, photon counting detector arrays are generally unable to process photon emission rates normally utilized in computed tomography scanners and/or other imaging modalities.